This invention relates to a heat recuperative fluid fuel burner with concentric burner tubes in which fuel and air in a rich fuel-air ratio is burned in the central burner tube and fuel and air in a lean fuel-air ratio is burned in the outer burner tube so as to reduce NOx emissions and improve flame luminosity when the flames are mixed together to form a final flame envelope.
The use of natural gas fuel and other gas fuels for heating purposes is less expensive than using oil or coal and other such fuels, but natural gas fuels tend to produce a flame with low luminosity. High flame luminosity is desirable to increase radiant heat transfer from the flame. In addition, as the temperature of the flame emitted from a gas-air mixture is increased, additional NOx emissions are created. If the combustion air is preheated prior to mixing with the fuel, the increased air temperature tends to reduce flame luminosity in conventional hot air burners and in conventional recuperative burners. The blue translucent gas flame is not desirable when radiation is the predominant heat transfer mechanism in high temperature applications. Therefore, conventional natural gas burners usually are considered to be impractical for use with high temperature applications such as in the primary metals industry and in the glass production industry where a high radiation component of heat transfer is desired.
It has been known to burn lean and rich fuel mixtures separately at low flame temperatures and to mix the flames after partial burning of the rich mixture has been accomplished so as to reduce NOx emissions in the resulting high temperature flame. Most of the existing low NOx burners with stage combustion are series type, in that, for example, a lean fuel-air mixture is injected into the first stage of a burner and burned, and additional fuel is added downstream of the first stage combustion zone. In other situations a rich fuel-air mixture is initially injected in the burner and additional air is injected downstream of the first stage combustion zone. This is a series arrangement in which the combustion products of the first stage are cooled moderately to keep the flame temperature down in order to reduce the possibility of thermal NOx formation. This tends to decrease the amount of free oxygen for possible reaction with nitrogen in the final flame because the utilization of oxygen for the rich and the lean flames reduces the amount of oxygen left for burning in the final flame. Generally, both the lean and rich flame temperatures should be maintained below 2,800.degree. F. in order to avoid excessive NOx emissions. Therefore, preheating the combustion air which results in a higher flame temperature creates the hazard that the flame will exceed the desired temperature.
If super lean and super rich fuel-air mixtures are used in staged combustion chambers, the flame temperatures can be kept below the desired temperatures for avoiding NOx emissions even if the combustion air is preheated. In the rich fuel-air chamber, NOx emissions are unlikely to form because of the small supply of free oxygen. The excess air in the lean fuel-air chamber allows more air to lower the flame temperature.